Like the unix xargs program. Useful for when we've got very long command
lines that might overflow an OS limit on command line length and so you
need to invoke a command multiple times to get all the args in.

Get the file path for this particular module. In the IO monad
because it looks for the actual file. Might eventually interface
with preprocessor libraries in order to correctly locate more
filenames.
Returns empty list if no such files exist.